The present invention relates generally to bandpass filters and is particularly directed to a switchable bandpass filter for processing a wide band RF signal particularly adapted for use in a frequency converter.
An electronic tuner such as used in atypical RF receiver includes up to three tunable circuits, i.e., a tunable circuit between the antenna input and the RF amplifier, a tunable circuit between the RF amplifier and the mixer, and a tunable circuit associated with the oscillator. Each of these tunable circuits is simultaneously tuned to a frequency representing the desired station, or channel. When more than one octave of frequencies is to be covered by the tuner, band switching techniques are frequently used to selectively change some elements of each tunable circuit in order to accommodate the entire desired frequency band.
Also, many tuners or frequency converters, such as for example those used with CATV systems, operate without a selective front end or mixer protection. As the desired RF bandwidth in this case increases over an octave of frequencies, the number of intermodulation products and the possibility of crossmodulation products quickly increase, imposing greater demands upon tuner.
Prior art approaches to multi-band tuning and mixer protection have generally involved the use of varactor diodes and/or switching diodes in the tuning stages. The varactor diode approach makes use of circuit elements that exhibit capacitances which are functions of the applied voltage. By varying the DC tuning voltage applied to a varactor diode, its capacitance changes, and the frequency to which the resonant circuit is tuned may be thereby adjusted within approximately one octave frequency range as desired. The limitation of the varactor tuning range to approximately one octave is overcome frequently by means of switching diodes forming a tuned and bandswitched network together with the varactor diodes and other mostly inductive components.
Another limitation to this varactor diode approach arises from an inherent characteristic of a varactor diode. Specifically, a varactor diode exhibits a nonlinear variation in capacitance as a function of the applied RF voltage. This non-linearity may add more crossmodulation and intermodulation products to that caused by active elements of the tuner.
Another problem inherent in varactor tuned frequency converters with a relatively high IF frequency (upconverter) is the difficulty of tracking the frequency between the RF and oscillator tuned circuits. To overcome this problem, complicated analog or digital schemes have been used.
On the other hand, a simple switching diode approach to band selection generally involves the use of separate bandpass filter sections that are selectively switched in and out of circuit depending upon the frequency band desired. In this switching diode approach, one bandpass filter section is generally active, while the remaining bandpass filter sections are switched out of circuit and are thus rendered inactive. Thus, the majority of circuit components are not in use at any given time and there is, consequently, a high component count for the various bandpass filter sections. In addition, more than one switching diode is generally required for each frequency band to allow for the switching in and out of circuit of both the input and the output of each bandpass filter section so as to select the circuit for receiving a desired frequency band.
In order to reduce the cross- and intermodulation products in wide band CATV tuners and signal converters, double balanced mixers are frequently employed. However, because the CATV band has expanded into the UHF band, the practical limits of double balanced mixer performance within the CATV tuner have essentially been reached in terms of tradeoff between intermodulation and noise performance. With this large RF bandwidth, mixer intermodulation and noise performance determine the dynamic range of the CATV tuner. The noise figure for a typical frequency converter chain with a mixer, which typically exhibits a conversion loss from 6 to 8 dB, is generally in the range of 10 to 13 dB. Placing a feedback type preamplifier with a gain of 10 dB in front of the mixer improves the system""s noise figure to 5 to 7 dB, but decreases the signal converter""s RF processing capability by an associated 10 dB. This decrease of the signal converter""s RF processing capability is generally unacceptable, except where an electronic band switch is also placed in front of the mixer state. However, this capability has not generally been available without employing overly complicated switching arrangements with large numbers of electronic components.
The invention disclosed in U.S. Pat. No. 4,571,560 overcomes one or more of the aforementioned limitations of the prior art by providing a switchable bandpass filter which is capable of operating over a large bandwidth, which employs a relatively low number of components by using many of the components in each of the inactive, or bandstop, sections of the filter, and which utilizes only a single switching diode in each bandpass filter section.
As disclosed in the aforementioned patent, one of the bandpass filter sections is activated by reverse biasing its corresponding switching diode. The other switching diodes are forward biased in order to deactivate their corresponding bandpass filter sections. These forward biased switching diodes, however, conduct in parallel, which causes a relatively large power drain. Unfortunately, in some battery powered applications, such as where tuners are used in laptop computers and in other portable devices, a large power drain shortens the time that the device can operate off of a single battery charge.
The present invention reduces the power drain in a bandpass filter by coupling the filter controlling switches in series rather than in parallel to thereby reduce power drain. Furthermore, the elements (e.g., tri-state buffers) that control the switches may be integrated together with other functional blocks of a tuner. For example, it is customary to integrate all sorts of switching means within a frequency synthesizer IC while using the control bus of the latter also for the purpose of bandswitching thus minimizing the cost of the control circuits.
In accordance with one aspect of the present invention, a switchable filter for passing one of a plurality of frequency bands comprises a filter input terminal, a filter output terminal, n bandpass filter sections, and n switches. Each of the n bandpass filter sections is coupled between the filter input terminal and the filter output terminal, and each of the n bandpass filter sections is arranged to pass a different frequency band. Each of the n switches is coupled to a respective one of then bandpass filter sections, and each of the n switches is operable for assuming either an OFF state for activating a corresponding one of the n bandpass filter sections or an ON state for deactivating the corresponding one of the n bandpass filter sections. The n switches are arranged and controlled so that, when any one of the n switches is operated to its OFF state, the remaining nxe2x88x921 switches are operated to their ON states and form a series circuit for conducting a common current, and n ∃ 3.
In accordance with another aspect of the present invention, a switchable filter for passing one of a plurality of frequency bands comprises a filter input terminal, a filter output terminal, at least first, second, and third bandpass filter sections, at least first, second, and third switches, and a controller. Each of the at least first, second, and third bandpass filter sections is coupled between the filter input terminal an the filter output terminal, and each of the at least first, second, and third bandpass filter sections is arranged to pass a different frequency band. The first switch is coupled in controlling relationship to the first bandpass filter section, the second switch is coupled in controlling relationship to the second bandpass filter section, and the third switch is coupled in controlling relationship to the third bandpass filter section. The controller is coupled to the at least first, second, and third switches. The controller controls the first switch to an OFF state so that the first bandpass filter section is activated and controls the second and third switches to ON states so that the second and third switches conduct in series so as to deactivate the second and third bandpass filter sections. The controller controls the second switch to an OFF state so that the second bandpass filter section is activated and controls the first and third switches to ON states so that the first and third switches conduct in series so as to deactivate the first and third bandpass filter sections. The controller controls the third switch to an OFF state so that the third bandpass filter section is activated and controls the first and second switches to ON states so that the fist and second switches conduct in series so as to deactivate the first and second bandpass filter sections.
In accordance with still another aspect of the present invention, a switchable filter for passing one of a plurality of frequency bands comprises a filter input terminal, a filter output terminal, first, second, third, and fourth bandpass filter sections, first, second, third, and fourth switches, and a controllers. The first bandpass filter section is coupled between the filter input terminal and the filter output terminal, and the first bandpass filter section is arranged to pass a first frequency band. The second bandpass filter section is coupled between the filter input terminal and the filter output terminal, and the second bandpass filter section is arranged to pass a second frequency band. The third bandpass filter section is coupled between the filter input terminal and the filter output terminal, and the third bandpass filter section is arranged to pass a third frequency band. The fourth bandpass filter section is coupled between the filter input terminal and the filter output terminal, and the fourth bandpass filter section is arranged to pass a fourth frequency band. The first, second, third, and fourth frequency bands are different. The first switch is coupled to the first bandpass filter section, the second switch is coupled to the second bandpass filter section, the third switch is coupled to the third bandpass filter section, and the fourth switch coupled to the fourth bandpass filter section. The controller is coupled to the first, second, third, and fourth switches. The controller controls the first switch so that the first bandpass filter section is activated and controls the second, third, and fourth switches so that the second, third, and fourth switches conduct current in series to deactivate the second, third, and fourth bandpass filter sections. The controller controls the second switch so that the second bandpass filter section is activated and controls the first, third, and fourth switches so that the first, third, and fourth switches conduct current in series to deactivate the first, third, and fourth bandpass filter sections. The controller controls the third switch so that the third bandpass filter section is activated and controls the first, second, and fourth switches so that the first, second, and fourth switches conduct current in series to deactivate the first, second, and fourth bandpass filter sections. The controller controls the fourth switch so that the fourth bandpass filter section is activated and controls the first, second, and third switches so that the first, second, and third switches conduct current in series to deactivate the first, second, and third bandpass filter sections.